Apparatus and method for determining weight loss of a heated material

ABSTRACT

The present invention provides a method and apparatus for accurately determining weight loss of a sample during heating in a furnace. The method includes the steps of placing a sample in a heated furnace, heating the sample while measurements of sample weight are made, determining rate function from the sample weight measurements, producing a weight loss correction factor using the rate function and using the weight loss correction factor to obtain a corrected weight loss for the sample.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional No.60/093,172, filed on Jul. 17, 1998, which is incorporated by referencein its entirety.

FIELD OF THE INVENTION

[0002] This invention is concerned with accurately weighing heatedmaterials, and is especially applicable to a pyrolysis furnace and tothe measurement of weight loss in such a furnace.

BACKGROUND OF THE INVENTION

[0003] Certain industrial processes require accurate measurement of theweight or mass of a material before it is in a state of thermalequilibrium. In some cases, it is necessary to achieve accuracy on theorder of tenths of a gram with samples larger than 3 kg. For example,the construction industry requires the measurement of asphalt contentfor quality control purposes. Asphalt is a mixture of asphalt binder andaggregate and is used heavily in the construction of roads. Themechanical properties of this mixture depend on many parameters, such asthe asphalt binder content by weight and the gradation of the aggregate.In order to measure the quality of these materials, the contractor needsa process to separate the binder from the aggregate.

[0004] In the past, there have been several accepted methods to obtainthis information. Two such methods involved chemical solvents andnuclear isotopes. The nuclear asphalt content gauge can be used toaccurately measure the binder content of asphalt in just a few minutes.Although this method is fast, the drawback is that gradation analysiscannot be obtained. Chemical solvents can give both asphalt content andgradation analysis. However this method is laborious, time consuming,and the waste solvent poses environmental problems.

[0005] In recent years, a method of igniting asphalt in order to measurethe weight loss due to combustion has become accepted. Although thismethod is relatively slow as compared to the nuclear techniques,gradation analysis can be obtained as soon as the ash has cooled. Withthe advent of new technologies in the construction industry, thestandards have also become more stringent. Variations in weight lossmeasurements from lab to production site to construction site, and evenfurnace manufacturer, must be minimized.

[0006] In the conventional industry process, a sample is weighed to thenearest tenth of a gram using an external scale and placed in a basketassembly. The assembly is then placed in a preheated furnace, which isoutfitted with an internal scale assembly or load cell. The door issecured, and the weighing process begins. During the first few moments,a tare or beginning weight is measured. During the next few minutes, theasphalt binder begins to burn and the furnace automatically calculates aweight loss relative to the initial weight and calculates the real timeasphalt binder content. The entire process may last from 20 to 60minutes depending on the initial sample weight and design of thefurnace.

[0007] Since the asphalt is usually mixed at a temperature of about 150°C., and the furnaces are usually preheated to temperatures near 538° C.,thermal instabilities exist that make the process of obtaining anaccurate initial weight of the asphalt a very challenging endeavor.Typically, the errors incurred are on the order of a few grams, anddecrease as the sample temperature approaches the temperature of thefurnace. The largest error in the weight loss determined using thismethod is due to an erroneous tare weight obtained during the first fewminutes. Generally, the internal scale in the furnace reports a higherbasket assembly weight during the first few minutes in the furnace thanone would obtain from an external weighing. This error is the directresult of the temperature differential between the furnace and thesample and basket assembly. Furthermore, the last few minutes in thefurnace atmosphere are measured as lighter in weight by the internalscale than one would expect externally. Compared to external scalemeasurements at ambient temperature, the furnace internal scaleoverestimates the actual weight loss of the sample.

[0008] There have been several attempts to clarify the physics of thiseffect. In one patent, U.S. Pat. No. 5,279,971 to Schneider, the initialerror in tare weight is reported as due to moisture absorbed in theasphalt. However, an asphalt plant mixes these constituents at 150° C.and moisture accounts for a small percent by weight, if any. Even wherethe sample is dried overnight and all moisture is removed, the sameerrors occur. The Schneider patent reports that samples should bepreheated to 300° C. before placing them in the 550° C. furnace. TheSchneider patent states that this reduces the “moisture” error.Actually, the error in tare weight was reduced only because thetemperature differential between the sample and furnace was 200° C. asopposed to 400° C.-500° C. with a typical sample removed from theproduction line.

[0009] The temperature error caused by placing a relatively cool sampleinto an extremely hot oven results in a complicated model involvingseveral external factors, such as air density, air flow, and bombardmentof the sample and pan assembly by high energy gas molecules.Furthermore, these factors affect the measurement in different waysaccording to the properties of the sample, such as mass, thermalcapacity, thermal conductance, voids or density, and specific gravity.Thus, there are many different combinations of these variables thatperturb the initial measurement. There remains a need in the art for amethod of accurately weighing samples in a heated furnace that takesinto account the complex effects of thermal instability present duringthe initial weighing process.

SUMMARY OF THE INVENTION

[0010] The present invention provides a method and apparatus foraccurately weighing samples in a heated furnace. More particularly, thepresent invention provides a method and apparatus in which the weightloss of a sample may be accurately determined as the sample is heated ina furnace. In one specific embodiment, the sample is an asphaltbinder/aggregate paving mix and the method and apparatus are utilized toaccurately measure the asphalt binder content of the paving mix bydetermining the weight loss resulting from pyrolysis of the asphaltbinder. Using the present invention, weight loss values calculated usingthe internal scales of a furnace are within about 0.05% of the weightloss values calculated with an external scale.

[0011] According to the invention, a correction factor is generatedwhich corrects for errors in the measurement of the tare weight of thesample due to external influences and variables such as those notedabove. The invention may additionally correct for errors in the endpoint weight, also due to external influences. A method in accordancewith the broad aspects of the invention, includes the steps of placing asample in a heated furnace, heating the sample while measurements ofsample weight are made, determining a rate function from the samplemeasurements, producing a weight loss correction factor using the ratefunction, and using the weight loss correction factor to obtain acorrected weight loss for the sample.

[0012] In another aspect, the method includes the steps of placing acombustible sample in a heated furnace, heating the sample whilemeasurements of the weight of the sample are made, determining a weightloss rate function from the sample weight measurements, determining theapproximate time at which the onset of sample combustion occurs,producing a weight loss correction factor using the time of combustiononset and the weight loss rate function, and using the weight losscorrection factor to obtain a corrected weight loss for the sample.

[0013] The weight loss rate function may be suitably determined from thesample weight measurements using regression analysis, such as leastsquares regression analysis, or other known techniques. During theinitial heating of the sample prior to combustion, the weight loss ratemay be suitably modeled by a linear function, although other functionscould be employed. The time at which the onset of sample combustionoccurs can be ascertained in a number of ways. In one embodiment oraspect, combustion onset may be determined by observing the time atwhich the weight loss rate ceases to be linear, or departs from linearby some threshold amount. In another embodiment or aspect, combustiononset may be determined by monitoring the rate of change in sampletemperature or combustion chamber temperature and determining therefromthe projected time at which the sample will reach a known combustiontemperature for the particular sample or some other selectedtemperature. Still another approach involves monitoring the rate ofchange in sample temperature or combustion chamber temperature anddetermining the time at which the temperature change rate ceases to belinear, or departs from linear by some threshold amount. Instead ofrelying upon combustion onset time, it is possible to use other values,such as combustion onset time less 10% or even a fixed time interval.The appropriate method of determining the combustion onset time or othervalue may depend, in part, on the design of the furnace.

[0014] The present invention also provides an apparatus for determiningweight loss of a sample, comprising a furnace, a scale mounted withinthe furnace for measuring sample weight, a data store operativelyconnected to said scale for storing sample weight measurements, and aweight loss correction factor generator for generating a weight losscorrection factor using the sample weight measurements in the datastore. The apparatus may also include means for generating a correctedweight loss measurement using a final sample weight measurement from thedata store and the weight loss correction factor.

[0015] Preferably, the weight loss correction factor generator comprisesmeans for determining a weight loss rate function from the sample weightmeasurements in the data store, means for determining the approximatetime of combustion onset, and means for generating a weight losscorrection factor using the time of combustion onset and the weight lossrate function.

[0016] Additional features and aspects of the invention will becomeapparent from the detailed description which follows and from theaccompanying drawings, which are intended to be illustrative of theinvention, but not restrictive as to the scope and breadth of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Having thus described the invention in general terms, referencewill now be made to the accompanying drawings, which are not necessarilydrawn to scale, and wherein:

[0018]FIG. 1 illustrates a preferred design of an analytical furnaceuseful in the present invention;

[0019]FIG. 2 illustrates a typical burn cycle for a 1,500 gram sample;

[0020]FIG. 3 illustrates the mass or weight loss profile and temperatureprofile of the first ten minutes of a typical burn cycle;

[0021]FIG. 4 illustrates the effect of heating on the weight of an emptypan assembly; and

[0022]FIG. 5 illustrates a preferred design of a pan assembly useful inthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

[0024]FIG. 1 illustrates an analytical furnace 10 with a combustionchamber 12 and a door 20 that provides access to the combustion chamber.The furnace 10 further includes a sample support 14 that is operativelyconnected to an internal scale assembly (not shown) that measures theweight of the sample during the combustion cycle. In one embodiment, thesupport 14 comprises a pair of rails positioned to receive a sample. Theinternal scale may be any known weighing device in the art, such as aload cell.

[0025] A heat transfer plate 16 may be placed above the sample support14. The sample, such as asphalt, is placed in a sample pan assembly 18and placed in the combustion chamber 12 such that the pan assembly restson the plate 16 during heating. A temperature sensor (not shown), suchas a thermocouple, is strategically located near the sample to measurechamber or sample temperature.

[0026] The plate 16 increases the transfer of heat into the sample andaids in preheating the air as it enters the chamber 12 through holesbelow the plate. To this end, the plate 16 is preferably made of amaterial having high thermal heat capacity and good heat conductivity.Particularly suitable are materials such as silicon carbide, aluminumoxide and some metals. Quick transfer of heat into the pan assembly 18assists in reducing the settling time and duration of thermalinstability.

[0027] Advantageously, a processor 26 is operatively connected to thefurnace 10. The processor 26 may be any computer hardware, software orcombination thereof capable of performing calculations and manipulatingdata as needed to practice the present invention. Preferably, theprocessor 26 includes one or more data stores for storing data such asweight and temperature readings. Additionally, the processor preferablyincludes a digital filter for filtering data measurements, such asweight and/or temperature measurements, to dampen or reduce oscillationsand noise caused by the mechanical vibrations of a thermally expandingsystem. The digital filter smoothes the response of the weight and/ortemperature data. The digital filter could be as simple as an N pole lowpass Butterworth-type filter, or even an adaptive filter as known to oneskilled in the art.

[0028] Typically, the furnace 10 is utilized to measure the weight of acombustible portion of a sample by measuring the weight of the samplebefore and after combustion of the combustible portion of the sample. Acommercially available furnace suitable for use with the presentinvention is Model 4155B available from Troxler Electronic Laboratories.As used herein, combustion refers generally to the boiling, evaporation,thermal degradation and thermal decomposition of the combustible portionof a sample.

[0029]FIG. 2 illustrates a typical burn cycle of a combustible samplewith time as the abscissa and weight loss in grams on the ordinate. Theburn cycle shows the weight loss that occurs during a combustion cycle,from the time the sample is placed in the furnace until combustion ofthe combustible portion of the sample is complete. At t=0, the sample isloaded into the chamber and the process begins. Point A indicates thebeginning of the burn cycle. Between t=0 and point A the system marks abeginning point commonly referred to as the tare. Point B marks the endof the flame, and point and C indicates the end of the cycle, asdetermined by a slope of less than 0.1 g per minute. In an automateddata collection system, the asphalt content by weight is calculatedusing the difference between the weight loss of point C and the tare.

[0030]FIG. 3 shows the expanded view of the burn cycle concentrating onthe first 10 minutes. The material weight loss is shown in FIG. 3A whilethe chamber temperature is illustrated in FIG. 3B. When the sample isfirst loaded into the chamber, the chamber temperature begins to drop.This is evident from the response between points A and B. Thetemperature then increases in a linear fashion to point C where apositive identification of the burn is evident by the deviation from thelinear response.

[0031] As shown in FIG. 3A, at t=0, the weight loss increases in alinear fashion up to point D. Between points D and E the functiondescribing loss deviates from a linear relationship. The ripple in theweight curve is due to oscillations in the pan assembly 18. During theinitial heating of this assembly 18, the pans may warp and begin to rockon the platform at a frequency determined by the rocking moment and massof this assembly and the sample.

[0032] It has been hypothesized that all the loss from t=0 to point E ofFIG. 3A was due to moisture evaporation from the sample. However, FIG. 4shows a similar weight loss response even with an empty pan assembly 18.Note that FIG. 4 is a graph of total weight rather than weight loss.This indicates that the weight loss during the first few minutes is aphenomenon linked with temperature instabilities and not moisture. Themoisture error theory lead to the practice of reading the weight lossdirectly off the curve of FIG. 3A at point E, as taught by U.S. Pat. No.5,279,971 to Schneider. Unfortunately, the weight loss obtained byinterpreting the correction factor exclusively by the weight indicatedby E also contains the boiling, evaporation and thermal degradation ofthe asphalt material. Thus, the Schneider method actually underestimatesthe weight of the asphalt binder by incorporating some of the weightlost due to combustion of the binder in the correction factor.

[0033] One of the largest effects pertaining to an inaccurate tare isdue to the difference in air densities and void content of the asphaltsample. During the initial linear portion of the weight loss curve, therelatively cold asphalt is out-gassing and becomes lighter. Thebombardment of the pan assembly 18 by energetic gas molecules creates atransfer of momentum or force, which also decreases as thermalequilibrium approaches. Furthermore, during this period the airflow andturbulence created in the combustion chamber 12 begins to settle. Whenthis process is near completion, evaporation of the petroleum-basedbitumen of the asphalt begins, as indicated by point D of FIG. 3A. Thisis the beginning of the nonlinear response of the weight loss curve.However, even though the outgassing has decreased by point D, it is notcomplete, but merely accompanied by evaporation up until the point ofcombustion located at point E.

[0034] As previously stated, a problem arises when the tare is obtainedbefore thermal stability has been achieved. Unfortunately, with acombustible sample, thermal stability is not achieved until after pointB in FIG. 2. Hence a weight loss correction factor, typically measuredin grams, is necessary to obtain the proper beginning weight.

[0035] The present invention provides a method of accurately determiningthe weight loss of a sample during heating, wherein the method includesusing a weight loss correction factor to obtain a corrected weight lossfor the sample. The method includes placing a sample, such as acombustible sample, in a heated furnace and heating the sample whilemeasurements of sample weight are made. The sample weight measurementsare used to determine a rate function. The rate function, in turn, isused to produce a weight loss correction factor. Thereafter, the weightloss correction factor may be used to obtain the corrected weight lossof the sample.

[0036] The sample weight measurements define a weight loss curve, suchas illustrated in FIG. 3A, wherein the curve includes an initialsubstantially linear portion and a subsequent non-linear portion.Preferably, the rate function is a weight loss rate function comprisinga function corresponding to the substantially linear portion of theweight loss curve. The weight loss rate function may be determined byapplying a regression analysis, such as a least squares regressionanalysis, to the sample weight measurements. In a preferred embodiment,the weight loss rate function comprises a linear function correspondingto the substantially linear portion of the weight loss curve.

[0037] Once the rate function is determined, the initial substantiallylinear portion of the weight loss curve may be linearly extrapolatedbeyond the linear portion of the weight loss curve. Since it is believedthe linear portion of the weight loss curve is mainly attributable toweight measurement errors caused by thermal instabilities as discussedabove, extrapolation of the linear portion of the weight loss curve tothe approximate point of combustion will provide a weight losscorrection factor that will negate the effect of thermal instability onthe measurement of weight loss of the sample. The remaining weight loss(occurring after the approximate onset of combustion) should beattributable to combustion of the combustible portion of the sample,such as asphalt binder.

[0038] Thus, in a preferred embodiment of the invention, the onset ofcombustion or approximate onset of combustion is determined in order toascertain the point at which changes in weight are no longerattributable to thermal instabilities present in the furnace. As shownin FIG. 3A, once this time is known, the weight loss correction factormay be calculated by extrapolating the rate function to that time.

[0039] For example, in one embodiment, when the approximate combustiontime has been determined, the calculated time is inserted into theweight loss rate function to determine the weight loss correctionfactor. Where a linear function is used, the time is multiplied by theslope of the linear equation derived from the substantially linearportion of the weight loss curve, and the intercept of the linearequation is added to this result to obtain the weight loss correctionfactor. Notice that this calculated value is much less than the measuredweight loss at this point in time, as the measured weight loss haspartially incorporated the combustion of the asphalt binder, or othercombustible portion, of the sample.

[0040] The approximate onset of combustion may be determined in a numberof ways. For example, as shown in FIG. 3A, the approximate onset ofcombustion results in a departure of the weight loss curve from a linearresponse. Thus, the onset of combustion may be determined by determiningthe time at which the weight loss rate departs from a linear function bya threshold amount. The threshold amount may vary from zero to anysuitable amount, such as about three grams. In other words, theapproximate point of combustion may be determined as the time at whichthe actual measured weight of the sample deviates from the extrapolatedweight calculated using the rate function by a threshold amount.

[0041] Similarly, the approximate onset of combustion may be determinedusing the temperature profile as shown in FIG. 3B. As shown, the chambertemperature or sample temperature departs from a linear function at theapproximate point of combustion. Thus, the onset of combustion may bedetermined by monitoring the rate of change in sample or combustionchamber temperature and determining the time at which the rate of changeof the monitored temperature departs from a linear function by athreshold amount. For example, a rate function for the substantiallylinear portion of the temperature curve may be calculated andextrapolated. The actual temperature of the sample or chamber may becompared to the extrapolated temperature calculated using the ratefunction and the onset of combustion may be determined as the time atwhich the two values diverge by a threshold amount, such as about 10° C.

[0042] Alternatively, the onset of combustion may be determined as thetime at which the sample temperature or combustion chamber temperaturereaches a predetermined temperature. Since asphalt typically ignites ata temperature of about 460° C., the onset of combustion may beapproximated by simply determining the time at which 460° C. is reachedin the combustion chamber and using that time, and the rate functiondiscussed above, to calculate the weight loss correction factor.Further, repeated experimentation with samples of a known initial weightusing the same furnace would enable the user to determine theappropriate onset of combustion time without reference to temperaturedata. For example, if several 1500 gram samples are burned in aparticular furnace, the user could estimate the time to combustion forthat sample size in that furnace type.

[0043] Once the weight loss correction factor is calculated, the finalcorrected weight loss of the sample may be calculated by subtracting, orotherwise applying, the weight loss correction factor from the finalmeasured weight loss of the sample. In this manner, the approximateweight loss attributable to thermal instability or other externalfactors is removed from the final weight loss calculation, resulting ingreater accuracy.

[0044] The integrity of this method is associated with the slope of theweight curve. In practice, small masses reach equilibrium at a fasterrate than larger masses. Likewise, different installations and systemswill achieve thermal equilibrium at differing rates. In this case, theresponse of the sample is related to the thermal capacity of thefurnace, the sample, and airflow of the installation. However, theseparameters are naturally accounted for through the slopes of thelinearized weight and temperature curves.

[0045] As explained above, a linear function is believed to adequatelymodel the weight loss due to thermal instability. However, otherfunctions known in the art could be used for the rate function. Forexample, an exponential function, such as Equation 1, is believed toaccurately model weight loss of a sample due to thermal instability.

Loss=A*EXP(Bt)+C   Equation 1

[0046] Equation 1 could be used to determine a weight loss correctionfactor without determining onset of combustion where a single type ormodel of furnace is utilized. If all furnaces that will utilize the ratefunction are of the same type, so that characteristics such as heat-uptime, airflow and physical size of the chamber and pan assembly are thesame, a statistical sampling of the furnaces can be used to determinethe exponential coefficient, B. Likewise, the manufacturer of aparticular furnace type could calculate the B coefficient for eachindividual furnace. For example, a non-combustible material could beinserted in the furnace and several weight loss versus time measurementscould be taken in order to determine the B coefficient.

[0047] Once the B coefficient is known for the particular furnace, onlytwo weight loss measurements are required in order to solve Equation 1for the remaining two constants, A and C. Once A and C are known, theweight loss correction factor may be calculated as the limit of Equation1 as t goes to infinity. Thus, using this method, the weight losscorrection factor is equal to the C constant.

[0048] When the pan assembly 18 is first placed into the chamber 12, itundergoes rapid expansion, which results in disproportionateinstabilities. Hence, a delay is preferably incorporated into the methodof the present invention to ignore this period. For example, a delay ofabout 20 to about 40 seconds may occur before sample weight measurementsare begun. The delay may be incorporated into the furnace 10 using, forexample, a delay timer incorporated into the processor 26. Following thedelay, sample weight measurements are begun.

[0049] Preferably, the sample or combustion chamber temperature is alsomonitored to determine when the minimum occurs. This allows furthersettling time to the pan assembly 18, and more importantly, generallysignals the beginning of the linear range of the chamber or sampletemperature curve. Preferably, temperature data collection begins afterthe chamber temperature reaches the minimum. Typically, the chambertemperature reaches the minimum value about two minutes after the sampleis placed in the furnace, but the time may vary depending on furnacetype. Once the temperature minimum occurs, both weight and temperaturedata are collected and preferably continue to be stored until the weightloss curve begins to become nonlinear. Typically, for small samples,data collection can occur up to about 4 or 5 minutes, while largermasses remain linear for as much as 6 to 8 minutes. Preferably, theweight data is collected for at least about two minutes to ensure thatsufficient data is taken to accurately determine the weight loss ratefunction. One way to detect when the weight curve becomes nonlinear isto calculate the residuals between the actual data and the correspondinglinear curve-fitted data. When the residuals become greater than somepredetermined value, then data collection is ceased. It is also possiblethat each curve (T and weight) are individually analyzed, as thetemperature response remains linear long after the weight has deviated.

[0050] The weight and temperature measurements discussed above, andcalculations utilizing those measurements, may be stored and manipulatedmanually or using processor 26. Preferably, the temperature and weightdata are fed into processor 26 and the processor performs allcalculations and curve-fitting functions.

[0051] A preferred design of the pan assembly 18 is shown in FIG. 5.During the data collection period, rapid thermal expansion takes placein the pan assembly 18. This expansion causes oscillations and reducesthe signal to noise ratio of the weight measurement. To reduce theseeffects, a preferred design of the pan assembly 18 incorporates amaterial with a low thermal expansion coefficient, such as stainlesssteel. The pan assembly 18 is also perforated to allow oxygen to flowinto the sample, and the lower catch pan 22 has a cross break to addmechanical strength and rigidity. The air gaps between the sample pans24 and catch pan 22 aid in oxidizing the asphalt while decreasing thetotal burn time. Preferably, the pan assembly 18 further includes aperforated top cover 28 and a bail strap 30 to hold the assembly inplace.

[0052] The present invention provides a method of measuring weight lossin an analytical furnace capable of consistently measuring weight lossregardless of the furnace type, thermal capacity of the furnace, thermalconductance and capacity of the sample, weight and void ratio of thesample, installation variances, the temperature difference between thesample and furnace and volume of the basket assembly.

[0053] Many modifications and other embodiments of the invention willcome to mind to one skilled in the art to which this invention pertainshaving the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is to beunderstood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

That which is claimed:
 1. A method of determining weight loss of a sample during heating, comprising: placing a sample in a heated furnace; heating the sample while making measurements of sample weight; determining a rate function from the sample weight measurements; producing a weight loss correction factor using the rate function; and using the weight loss correction factor to obtain a corrected weight loss for the sample.
 2. A method according to claim 1, wherein said step of determining the rate function comprises applying a regression analysis to the sample weight measurements.
 3. A method according to claim 1, wherein the sample weight measurements define a weight loss curve having an initial substantially linear portion and a subsequent non-linear portion, and wherein said step of determining the rate function comprises defining a linear function corresponding to the substantially linear portion of the weight loss curve.
 4. A method according to claim 1, wherein the sample weight measurements define a weight loss curve, and wherein said step of determining the rate function comprises defining an exponential function corresponding to the weight loss curve.
 5. A method according to claim 1, wherein said placing step comprises placing a combustible sample in a heated furnace.
 6. A method of determining weight loss of a sample during heating, comprising: placing a combustible sample in a heated combustion chamber of a furnace; heating the sample while making measurements of the weight of the sample; determining a weight loss rate function from the sample weight measurements; determining the approximate time at which the onset of sample combustion occurs; producing a weight loss correction factor using the combustion onset time and the weight loss rate function; and using the weight loss correction factor to obtain a corrected weight loss for the sample.
 7. A method according to claim 6, wherein said step of determining the weight loss rate function comprises applying a regression analysis to the sample weight measurements.
 8. A method according to claim 6, wherein the step of determining the approximate time at which the onset of combustion occurs comprises determining the time at which the weight loss rate departs from a linear function by a threshold amount.
 9. A method according to claim 6, wherein the step of determining the approximate time at which the onset of combustion occurs comprises: monitoring the rate of change in sample temperature or combustion chamber temperature; and determining the time at which the rate of change in the monitored temperature departs from a linear function by a threshold amount.
 10. A method according to claim 6, wherein the step of determining the approximate time at which the onset of combustion occurs comprises determining the time at which the sample temperature or the combustion chamber temperature reaches a predetermined temperature.
 11. A method according to claim 6, wherein the sample weight measurements define a weight loss curve having an initial substantially linear portion and a subsequent non-linear portion, and wherein said step of determining the weight loss rate function comprises defining a linear function corresponding to the substantially linear portion of the weight loss curve.
 12. A method of determining weight loss of a sample during heating, the method comprising: providing a sample having a combustible portion; heating the sample to a temperature and for a time sufficient to remove the combustible portion of the sample; measuring and recording the weight of the sample during said heating step to define a weight loss curve; defining from the recorded weight measurements a linear weight loss function corresponding to an initial substantially linear portion of the weight loss curve; determining the approximate time at which the onset of combustion occurs; extrapolating the linear weight loss function to said combustion onset time; determining a weight loss correction factor based on the extrapolated weight loss function.
 13. A method according to claim 12, further comprising the steps of: determining a measured weight loss of the sample after removal of the combustible portion; and generating a corrected weight loss by applying the weight loss correction factor to the measured weight loss.
 14. A method according to claim 12, wherein said heating step comprises heating the sample in a combustion chamber of a furnace.
 15. A method according to claim 14, wherein said step of determining the approximate time at which the onset of combustion occurs comprises: measuring the temperature of the sample or the combustion chamber during the heating step, the temperature measurements defining a temperature curve representing the measured temperature in relation to time, the curve comprising a substantially linear portion and a subsequent non-linear portion; linearly extrapolating the substantially linear portion of the temperature curve; determining the time at which the measured temperature diverges from the corresponding linearly extrapolated temperature by a predetermined amount.
 16. A method according to claim 14, wherein said step of determining the approximate time at which the onset of combustion occurs comprises determining the time at which the sample or combustion chamber temperature reaches a predetermined temperature.
 17. A method according to claim 12, wherein said step of determining the approximate time at which the onset of combustion occurs comprises determining the time at which the measured weight loss of the sample diverges from the corresponding linearly extrapolated weight loss function by a predetermined amount.
 18. A method according to claim 12, wherein said step of measuring the weight of the sample begins at least about 20 seconds after said heating step begins.
 19. A method according to claim 12, wherein the step of measuring the weight of the sample comprises digitally filtering weight measurements.
 20. A method according to claim 19, wherein the step of digitally filtering the weight measurements comprises filtering the measurements through a low pass filter.
 21. A method of determining weight loss of an asphalt sample during heating, the method comprising: providing an asphalt sample comprising asphalt binder and aggregate; heating the sample to a temperature and for a time sufficient to remove the asphalt binder; measuring and recording the weight of the asphalt sample during said heating step, the sample weight measurements defining a weight loss curve having an initial substantially linear portion and a subsequent non-linear portion; defining from the recorded weight measurements a linear weight loss function corresponding to an initial substantially linear portion of the weight loss curve; determining the approximate time at which the onset of asphalt binder combustion occurs; extrapolating the linear weight loss function to said combustion onset time; determining a weight loss correction factor based on the extrapolated weight loss function; determining a measured weight loss of the asphalt sample after removal of the asphalt binder; and generating a corrected weight loss by applying the weight loss correction factor to the measured weight loss.
 22. A method according to claim 21, wherein said heating step comprises: placing the asphalt sample in a sample pan; and placing the sample pan in a combustion chamber of a furnace.
 23. An apparatus for determining weight loss of a sample, comprising: a furnace; a scale mounted within said furnace for measuring sample weight; a data store operatively connected to said scale for storing sample weight measurements; and a weight loss correction factor generator for generating a weight loss correction factor using the sample weight measurements in said data store.
 24. An apparatus according to claim 23, further comprising means for generating a corrected weight loss measurement using a final sample weight measurement from said data store and said weight loss correction factor.
 25. An apparatus according to claim 23, wherein said weight loss correction factor generator comprises: means for determining a weight loss rate function from the sample weight measurements in said data store; means for determining the approximate time at which the onset of sample combustion occurs; and means for generating a weight loss correction factor using the approximate time of combustion onset and the weight loss rate function.
 26. An apparatus according to claim 25, wherein said means for determining the combustion onset time comprises means for determining the time at which the weight loss of the sample departs from a linear function by a threshold amount using the weight measurements in said data store.
 27. An apparatus according to claim 25, wherein said means for determining the combustion onset time comprises: a temperature sensor mounted within said combustion chamber, said temperature sensor operatively positioned to measure sample or combustion chamber temperature; a second data store operatively connected to said temperature sensor for storing temperature measurements; and means for determining the time at which the rate of change in the stored temperature measurements departs from a linear function by a threshold amount using the measurements in said second data store.
 28. An apparatus according to claim 25, wherein said means for determining the combustion onset time comprises: a temperature sensor mounted within said combustion chamber, said temperature sensor operatively positioned to measure sample or combustion chamber temperature; a second data store operatively connected to said temperature sensor for storing temperature measurements; and means for determining the time at which the measured temperature reaches a predetermined temperature using the measurements in said second data store.
 29. An apparatus according to claim 23, further comprising a delay timer operatively connected to said scale to delay the measurement of sample weight for a predetermined amount of time.
 30. An apparatus according to claim 23, further comprising a digital filter operatively connected to said data store for filtering sample weight measurements.
 31. An apparatus according to claim 30, wherein said digital filter comprises a low pass filter.
 32. An apparatus according to claim 23, further comprising a plate overlying said scale.
 33. An apparatus according to claim 23, further comprising a pan assembly operatively positioned for weighing on said scale, said pan assembly comprising a catch pan having a cross break and at least one sample pan overlying said catch pan.
 34. An apparatus according to claim 23, wherein said furnace comprises: a combustion chamber; and a door providing access to said combustion chamber.
 35. An apparatus for determining weight loss of a sample, comprising: a furnace, said furnace comprising a combustion chamber and a door providing access to said combustion chamber; a scale mounted within said furnace for measuring sample weight; a data store operatively connected to said scale for storing sample weight measurements; means for determining a weight loss rate function from the sample weight measurements in said data store; means for determining the approximate time at which the onset of sample combustion occurs; and means for generating a weight loss correction factor using the approximate time of combustion onset and the weight loss rate function.
 36. An apparatus according to claim 35, further comprising: a temperature sensor mounted within said combustion chamber, said temperature sensor operatively positioned to measure sample or combustion chamber temperature; and a second data store operatively connected to said temperature sensor for storing temperature measurements. 